[The Saucepan Man]

Well, one similarity is that they were both rendered incapable of properly defending their people. In Denethor's case, this was caused by despair at the strength of the enemy facing him, fuelled by his use of the Palantir which exposed him to Sauron's deceits. In Theoden's case, he was seduced into inaction by Wormtongue's words. The difference, of course, is that Gandalf was able to rouse Theoden from inaction, whereas Denethor's despair only deepened.

So, I suppose you could say that attempts were made by the enemy to neutralise both of them (and therefore their armies) - Theoden by Saruman through Wormtongue and Denethor by Sauron through the Palantir.